Medial Temporal Lobe Resection Research Articles

Memory functioning after hippocampal removal: Does side matter?

To address the memory functioning after medial temporal lobe (MTL) surgery for refractory epilepsy and relationships with the side of the hippocampal removal, 22 patients with pharmaco-resistant epilepsy who had undergone MTL resection (10 right/12 left) at the Salpêtrière Hospital were compared with 21 matched healthy controls. We designed a specific neuropsychological binding memory test that specifically addressed hippocampal cortex functioning, and left-right material-specific lateralization. Our results showed that both left and right mesial temporal lobe removal cause a severe memory impairment, for both verbal and visual material. The removal of left medial temporal lobe causes worse memory impairment than the right removal regardless of the stimuli type (verbal or visual) questioning the theory of the hippocampal material-specific lateralization. The present study provided new evidence for the role of both hippocampus and surrounding cortices in memory-binding whatever the material type and also suggested that a left MTL removal is more deleterious for both verbal and visual episodic memory in comparison with right MTL removal.

Effects of left and right medial temporal lobe resections on hemodynamic correlates of negative and neutral scene processing.

Enhanced visual cortex activation by negative compared to neutral stimuli is often attributed to modulating feedback from the amygdala, but evidence from lesion studies is scarce, particularly regarding differential effects of left and right amygdala lesions. Therefore, we compared visual cortex activation by negative and neutral complex scenes in an event‐related fMRI study between 40 patients with unilateral temporal lobe resection (TLR; 19 left [lTLR], 21 right [rTLR]), including the amygdala, and 20 healthy controls. We found preserved hemodynamic emotion modulation of visual cortex in rTLR patients and only subtle reductions in lTLR patients. In contrast, rTLR patients showed a significant decrease in visual cortex activation irrespective of picture content. In line with this, healthy controls showed small emotional modulation of the left amygdala only, while their right amygdala was activated equally by negative and neutral pictures. Correlations of activation in amygdala and visual cortex were observed for both negative and neutral pictures in the controls. In both patient groups, this relationship was attenuated ipsilateral to the TLR. Our results support the notion of reentrant mechanisms between amygdala and visual cortex and suggest laterality differences in their emotion‐specificity. While right medial temporal lobe structures including the amygdala seem to influence visual processing in general, the left medial temporal lobe appears to contribute specifically to emotion processing. Still, effects of left TLR on visual emotion processing were relatively subtle. Therefore, hemodynamic correlates of visual emotion processing are likely supported by a distributed cerebral network, challenging an amygdalocentric view of emotion processing.

Novelty processing depends on medial temporal lobe structures

ObjectivesThe goal of the present study was to identify the role of the medial temporal lobe (MTL) in the detection and later processing of novelty. MethodsTwenty-one epilepsy patients with unilateral MTL resection (10 left-sided; 11 right-sided) and 26 matched healthy controls performed an adapted visual novelty oddball task. In this task two streams of stimuli were presented on the left and right of fixation while the patients’ electroencephalogram was measured. The participants had to respond to infrequent target stimuli, while ignoring frequent standard, and infrequent novel stimuli that were presented to the left or right, appearing either contra- or ipsilateral to the patients’ resections. ResultsNovelty detection, as indexed by the N2 ERP component elicited by novels, was reduced by the MTL resections, as evidenced by a smaller N2 for patients than healthy controls. Later processing of novels, as indexed by the novelty P3 ERP component, was reduced for novels presented contra- versus ipsilateral to the resected side. Moreover, at a frontal electrode site, the N2-P3 complex showed reduced novelty processing in patients with MTL resections compared to healthy controls.The ERP differences were specific for the novel stimuli, as target processing, as indexed by the P3b, was unaffected in the patients: No P3b differences were found between targets presented ipsi- or contralaterally to the resected side, nor between patients and healthy controls. ConclusionsThe current results suggest that MTL structures play a role in novelty processing. In contrast, target processing was unaffected by MTL resections.

Early volumetric changes of hippocampus and medial prefrontal cortex following medial temporal lobe resection.

Previous studies have shown that cognitive demands and physical exercise stimulate adult neurogenesis in the dentate gyrus and hippocampus. Recent observations in healthy humans and patients with mild cognitive impairment moreover suggest that training-induced increases in hippocampal volume may be associated with improved memory performance. The corresponding plasticity processes in hippocampal volume may occur on timescales of months to years. For patients with focal lesions in this region, previous functional imaging studies suggest that increased recruitment of the contralateral hippocampus and extratemporal regions may be an important part of the reorganization of episodic memory. However, it is currently unclear whether focal damage to the medial temporal lobe (MTL) induces gray matter (GM) volume changes in the intact contralateral hippocampus and in connected network regions on a shorter timescale. We therefore investigated whether unilateral resection of the MTL, including the hippocampus, induces measurable volumetric changes in the contralateral hippocampus and in the default mode network (DMN). We recruited 31 patients with unilateral left (N=19) or right (N=12) hippocampal sclerosis undergoing MTL resection for treatment of drug-resistant epilepsy. Structural MRI was acquired immediately before and 3months after surgery. Longitudinal voxel-based morphometry (VBM) analysis revealed a significant increase of right hippocampal volume following resection of the left anterior MTL. Furthermore, this patient group showed GM volume increases in the DMN. These results demonstrate significant structural plasticity of the contralateral hippocampus, even in patients with a long-standing unilateral hippocampal dysfunction and structural reorganization processes extending to distant, but functionally connected brain regions.

Neural basis of episodic memory in the intermediate term after medial temporal lobe resection.

How the brain supports intermediate-term preservation of memory in patients who have undergone unilateral medial temporal lobe resection (MTLR) has not yet been demonstrated. To understand the neural basis of episodic memory in the intermediate term after surgery for temporal lobe epilepsy (TLE), the authors investigated the relationship between the activation of the hippocampus (HIP) during successful memory encoding and individual memory capacity in patients who had undergone MTLR. They also compared hippocampal activation with other parameters, including structural volumes of the HIP, duration of illness, and age at seizure onset. Thirty-five adult patients who had undergone unilateral MTLR at least 1 year before recruiting and who had a favorable seizure outcome were enrolled (17 left MTLR, 18 right MTLR; mean follow-up 6.31 ± 2.72 years). All patients underwent a standardized neuropsychological examination of memory function and functional MRI scanning with a memory-encoding paradigm of words and figures. Activations of the HIP during successful memory encoding were calculated and compared with standard neuropsychological memory scores, hippocampal volumes, and other clinical variables. Greater activation in the HIP contralateral to the side of the resection was related to higher postoperative memory scores and greater postoperative memory improvement than the preoperative baseline in both patient groups. Specifically, postoperative verbal memory performance was positively correlated with contralateral right hippocampal activation during word encoding in the left-sided surgery group. In contrast, postoperative visual memory performance was positively correlated with contralateral left hippocampal activation during figure encoding in the right-sided surgery group. Activation of the ipsilateral remnant HIP was not correlated with any memory scores or volumes of the HIP; however, it had a negative correlation with the seizure-onset age and positive correlation with the duration of illness in both patient groups. For the first time, a neural basis that supports effective intermediate-term episodic memory after unilateral MTLR has been characterized. The results provide evidence that engagement of the HIP contralateral rather than ipsilateral to the side of resection is responsible for effective memory function in the intermediate term (> 1 year) after surgery in patients who have undergone left MTLR and right MTLR. Engagement of the material-specific contralesional HIP, verbal memory in the left-sided surgery group, and visual memory in the right-sided surgery group were observed.

Characterization of brain network supporting episodic memory in the absence of one medial temporal lobe.

How the brain supports normal episodic memory function without medial temporal lobe (MTL) structures has not been well characterized, which could provide clues for new therapeutic targets for people with MTL dysfunction‐related memory impairment. To characterize brain network supporting effective episodic memory function in the absence of unilateral MTL, we investigated the whole‐brain cortical interactions during functional magnetic resonance imaging memory encoding paradigms of words and figures in patients who showed a normal range of memory capacity following unilateral MTL resection and healthy controls (HC). Compared to the HC, the patients showed less activation in the left inferior frontal areas and right thalamus together with greater activation in the many cortical areas including the medial prefrontal cortex (mPFC). Task‐based functional connectivity (FC) analysis revealed that the mPFC showed stronger interactions with widespread brain areas in both patient groups, including the hippocampus contralateral to the resection. Moreover, the strength of the mPFC FC predicts the individual memory capacity of the patients. Our data suggest that hyperconnectivity of distributed brain areas, especially the mPFC, is a neural mechanism for memory function in the absence of one MTL.

Decision-making impairments following insular and medial temporal lobe resection for drug-resistant epilepsy.

Besides the prefrontal cortex, the insula and medial structures of the temporal lobe are thought to be involved in risky decision-making. However, their respective contributions to decision processes remain unclear due to the lack of studies involving patients with isolated insular damage. We assessed adult patients who underwent resection of the insula (n = 13) or of the anterior temporal lobe (including medial structures) (n = 13) as part of their epilepsy surgery, and a group of healthy volunteers (n = 20), on the Iowa Gambling Task (IGT) and on the Cups Task. Groups were matched on sociodemographic, estimated-IQ and surgery-related factors. On the IGT, patients with temporal lobe resection performed significantly worse than both the insular and healthy control groups, as they failed to learn which decks were advantageous on the long-term. On the Cups Task, the insular and temporal groups both showed impaired sensitivity to expected value in the loss domain, when compared with healthy controls. These findings provide clinical evidence that the insula and mesiotemporal structures are specifically involved in risky decision-making when facing a potential loss, and that temporal structures are also involved in learning the association between behavior and consequences in the long-term.

The feeling of familiarity for music in patients with a unilateral temporal lobe lesion: A gating study

Previous research has indicated that the medial temporal lobe (MTL), and more specifically the perirhinal cortex, plays a role in the feeling of familiarity for non-musical stimuli. Here, we examined contribution of the MTL to the feeling of familiarity for music by testing patients with unilateral MTL lesions. We used a gating paradigm: segments of familiar and unfamiliar musical excerpts were played with increasing durations (250, 500, 1000, 2000, 4000ms and complete excerpts), and participants provided familiarity judgments for each segment. Based on the hypothesis that patients might need longer segments than healthy controls (HC) to identify excerpts as familiar, we examined the onset of the emergence of familiarity in HC, patients with a right MTL resection (RTR), and patients with a left MTL resection (LTR). In contrast to our hypothesis, we found that the feeling of familiarity was relatively spared in patients with a right or left MTL lesion, even for short excerpts. All participants were able to differentiate familiar from unfamiliar excerpts as early as 500ms, although the difference between familiar and unfamiliar judgements was greater in HC than in patients. These findings suggest that a unilateral MTL lesion does not impair the emergence of the feeling of familiarity. We also assessed whether the dynamics of the musical excerpt (linked to the type and amount of information contained in the excerpts) modulated the onset of the feeling of familiarity in the three groups. The difference between familiar and unfamiliar judgements was greater for high than for low-dynamic excerpts for HC and RTR patients, but not for LTR patients. This indicates that the LTR group did not benefit in the same way from dynamics. Overall, our results imply that the recognition of previously well-learned musical excerpts does not depend on the integrity of either right or the left MTL structures. Patients with a unilateral MTL resection may compensate for the effects of unilateral damage by using the intact contralateral temporal lobe. Moreover, we suggest that remote semantic memory for music might depend more strongly on neocortical structures rather than the MTL.

H.M.'s contributions to neuroscience: A review and autopsy studies

H.M., Henry Molaison, was one of the world's most famous amnesic patients. His amnesia was caused by an experimental brain operation, bilateral medial temporal lobe resection, carried out in 1953 to relieve intractable epilepsy. He died on December 2, 2008, and that night we conducted a wide variety of in situ MRI scans in a 3 T scanner at the Massachusetts General Hospital (Mass General) Athinoula A. Martinos Center for Biomedical Imaging. For the in situ experiments, we acquired a full set of standard clinical scans, 1 mm isotropic anatomical scans, and multiple averages of 440 μm isotropic anatomical scans. The next morning, H.M.'s body was transported to the Mass General Morgue for autopsy. The photographs taken at that time provided the first documentation of H.M.'s lesions in his physical brain. After tissue fixation, we obtained ex vivo structural data at ultra-high resolution using 3 T and 7 T magnets. For the ex vivo acquisitions, the highest resolution images were 210 μm isotropic. Based on the MRI data, the anatomical areas removed during H.M.'s experimental operation were the medial temporopolar cortex, piriform cortex, virtually all of the entorhinal cortex, most of the perirhinal cortex and subiculum, the amygdala (except parts of the dorsal-most nuclei-central and medial), anterior half of the hippocampus, and the dentate gyrus (posterior head and body). The posterior parahippocampal gyrus and medial temporal stem were partially damaged. Spared medial temporal lobe tissue included the dorsal-most amygdala, the hippocampal-amygdalo-transition-area, ∼2 cm of the tail of the hippocampus, a small part of perirhinal cortex, a small portion of medial hippocampal tissue, and ∼2 cm of posterior parahippocampal gyrus. H.M.'s impact on the field of memory has been remarkable, and his contributions to neuroscience continue with a unique dataset that includes in vivo, in situ, and ex vivo high-resolution MRI.

Enhanced Visual Cortical Activation for Emotional Stimuli is Preserved in Patients with Unilateral Amygdala Resection

Emotionally arousing pictures induce increased activation of visual pathways relative to emotionally neutral images. A predominant model for the preferential processing and attention to emotional stimuli posits that the amygdala modulates sensory pathways through its projections to visual cortices. However, recent behavioral studies have found intact perceptual facilitation of emotional stimuli in individuals with amygdala damage. To determine the importance of the amygdala to modulations in visual processing, we used functional magnetic resonance imaging to examine visual cortical blood oxygenation level-dependent (BOLD) signal in response to emotionally salient and neutral images in a sample of human patients with unilateral medial temporal lobe resection that included the amygdala. Adults with right (n = 13) or left (n = 5) medial temporal lobe resections were compared with demographically matched healthy control participants (n = 16). In the control participants, both aversive and erotic images produced robust BOLD signal increases in bilateral primary and secondary visual cortices relative to neutral images. Similarly, all patients with amygdala resections showed enhanced visual cortical activations to erotic images both ipsilateral and contralateral to the lesion site. All but one of the amygdala resection patients showed similar enhancements to aversive stimuli and there were no significant group differences in visual cortex BOLD responses in patients compared with controls for either aversive or erotic images. Our results indicate that neither the right nor left amygdala is necessary for the heightened visual cortex BOLD responses observed during emotional stimulus presentation. These data challenge an amygdalo-centric model of emotional modulation and suggest that non-amygdalar processes contribute to the emotional modulation of sensory pathways.

A neuropsychological approach to time estimation

Time estimation, within a range of seconds, involves cognitive functions which depend on multiple brain regions. Here we report on studies investigating the reproduction and production of three durations (5, 14, and 38 seconds) in four groups of patients. The amnesic patient underproduced the length of the long durations because of episodic memory deficit following bilateral medial temporal lesions. Epileptic patients (n = 9) with right medial temporal lobe resections underproduced the three durations because of a distorted representation of time in long-term memory. Traumatic brain injury patients (n = 15) made more variable duration productions and reproductions because of working memory deficits following frontal-lobe dysfunction. Patients with Parkinson's disease (n = 18) overproduced the short duration and underproduced the long duration because of a possible increase in internal clock speed following levodopa treatment, as well as working memory deficits associated with frontal-lobe damage. Further research, in neurological and psychiatric patients, is required to better understand the underlying mechanisms of time estimation.

Impaired recognition of fear in voices and reduced anxiety after unilateral temporal lobe resection

It has been reported that bilateral amygdala damage in humans compromises the recognition of fear and anger in nonverbal vocalizations (Scott et al., 1997). We addressed the possibility that unilateral temporal lobe damage might be sufficient to impair fear recognition in voices. For this purpose, we tested patients after left (n=10) or right (n=8) medial temporal lobe resection for the relief of intractable epilepsy using a set of nonverbal vocalizations (Belin, Fillion-Bilodeau, & Gosselin, 2008). To focus more narrowly on the role of amygdala subparts, we differentiated patients with complete amygdala damage vs. damage limited to the basolateral complex of the amygdala. The results confirmed for the first time that unilateral amygdala lesion including the basolateral complex can selectively impair recognition of fear and surprise expressed by voices, supporting the notion that the amygdala is a multimodal structure. Interestingly, this impairment was observed in patients with incomplete resection of the amygdala that spared the central nucleus and the corticomedial complex, suggesting that a resection of the basolateral complex is sufficient to affect fear recognition. Given that fear has often been considered as a precursor of anxiety, we also investigated the effect of such lesions on self-reported anxiety. The same patients appeared to be less anxious than control participants in their mood questionnaires. The association of impaired fear perception and decreased anxiety level is considered in the light of recent human and animal data, providing support for a neurobiological basis of mood changes in patients with unilateral temporal lobe damage.

The effects of medial temporal lobe resections on verbal threat and fear conditioning

A left hemisphere advantage in the processing of verbal threat has previously been reported, whereas both hemispheres seem equally important in fear conditioning. Here, we compared the effects of unilateral medial temporal lobe (MTL) resections on verbal threat as well as delay and trace fear conditioning. During verbal threat, right and left MTL-resections attenuated fear potentiated startle in comparison with controls. In contrast to previous studies, MTL-resections did not attenuate delay conditioning of skin conductance responses. Left and right resectioned patients did not differ in psychophysiological responses to verbal threat or delay fear conditioning. Trace conditioning was not observed in any group. Results suggest a bilateral MTL hemispheric involvement in the processing of verbal threat, whereas one intact hemisphere seems sufficient for delay conditioning.

Resection of the Medial Temporal Lobe Disconnects the Rostral Superior Temporal Gyrus from Some of its Projection Targets in the Frontal Lobe and Thalamus

Auditory memory in the monkey does not appear to extend beyond the limits of working memory. It is therefore surprising that this ability is impaired by medial temporal lobe (MTL) resections, because such lesions spare working memory in other sensory modalities. To determine whether MTL ablations might have caused the auditory deficit through inadvertent transection of superior temporal gyrus (STG) projections to its downstream targets, and, if so, which targets might have been compromised, we injected anterograde tracer (biotinylated dextran amine) in the STG of both the normal and MTL-lesioned hemispheres of split-brain monkeys. Interhemispheric comparison of label failed to show any effect of the MTL ablation on efferents from caudal STG, which projects to the inferior prefrontal convexity. However, the ablation did consistently interrupt the normally dense projections from rostral STG to both the ventral medial prefrontal cortex and medial thalamic nuclei. The findings support the possibility that the auditory working memory deficit after MTL ablation is due to transection of downstream auditory projections, and indicate that the candidate structures for mediating auditory working memory are the ventral medial prefrontal cortical areas, the medial thalamus, or both.

Characteristics and surgical outcomes for medial temporal post-traumatic epilepsy

A common post-traumatic location of epileptogenesis is the medial temporal lobe despite evidence of associated diffuse or remote cerebral injury. We undertook a review of post-traumatic medial temporal lobe epilepsy (MTLE) patients as part of an overall post-traumatic epilepsy population to assess the extent of cerebral injury sustained by this subpopulation and to establish whether surgical outcome differed from that of a non-traumatically-induced epilepsy population. A retrospective review of 57 patients operated for post-traumatic epilepsy (PTE) over a 10-year period (1993 – 2003) was undertaken with particular attention to those undergoing medial temporal resection. Preoperative magnetic resonance imaging (MRI) was assessed for the type and location of abnormalities. Postoperative outcomes were compared with those of patients with MTLE of non-traumatic origin operated by the same surgeon. Of the 57 patients operated, 30 cases underwent medial temporal lobe resection. The most common mechanism of injury was blunt trauma attributable to motor vehicle accidents with imaging abnormalities characterized by medial temporal sclerosis (MTS; 16 cases), T2/FLAIR hyperintensities (nine cases), periventricular gliosis (seven cases), diffuse cerebral atrophy (five cases) and focal encephalomalacia (three cases). Six patients had normal MRI studies. No significant differences in postoperative outcomes were found between post- and non-traumatic MTLE epilepsy groups. The presence of histopathological change in the medial temporal lobe varied greatly and provided no indication of a favourable postoperative outcome. Patients with post-traumatic medial temporal lobe epilepsy respond favourably to surgical treatment. In the case of medial temporal sclerosis, there is substantial variation of histopathological findings which correlate poorly with current imaging applications. The favourable outcomes obtained following surgery in this group attest to a commonality with other risk factors in the genesis of epilepsy in this location.

Autonoetic Consciousness in Autobiographical Memories after Medial Temporal Lobe Resection

This study aims to investigate autonoetic consciousness associated with episodic autobiographical memory in patients who had undergone unilateral medial temporal lobe resection for intractable epilepsy. Autonoetic consciousness, defined as the conscious feeling of mentally travelling back in time to relive a specific event, was assessed using the Remember/Know (R/K) paradigm across different time periods as proposed in the autobiographical memory task developed by Piolino et al. (TEMPau task). Results revealed that the two patient groups (left and right temporal resection) gave reduced sense of reliving (R) responses and more familiarity (K) responses than healthy controls. This poor autonoetic consciousness was highlighted when patients were asked to justify their Remember responses by recalling sensory-perceptive, affective or spatiotemporal specific details across all life periods. These results support the bilateral MTL contribution to episodic autobiographical memory covering the entire lifespan, which is consistent with the multiple trace theory of MTL function [7,9]. This study also demonstrates the bilateral involvement of MTL structures in recalling specific details of personal events characterized by autonoetic consciousness.

Medial temporal-lobe damage and memory for emotionally arousing odors

Recently, we found that healthy young adults remember odors leading to large emotional reactions better than odors provoking smaller emotional reactions. Because the amygdala is believed to be critically implicated in memory for emotionally arousing information and because it is part of the primary olfactory area, we hypothesized that patients with a unilateral medial temporal-lobe resection including the amygdala would not show enhanced memory for arousing compared to nonarousing odors. We tested odor memory in 19 patients (10 left, 9 right) who had undergone a unilateral medial temporal-lobe resection including the amygdala (MTLR) for treatment of intractable epilepsy and 19 healthy control subjects. Healthy individuals and patients with left or right MTLR showed comparable subjective emotional reactions to odors. Similarly, healthy individuals and patients with MTLR remembered unpleasant odors better than pleasant ones. However, unlike healthy individuals, patients with MTLR did not show better memory for emotionally arousing odors compared to nonarousing ones. Patients undergoing a MTLR, whether in the left or right hemisphere, lose the specific memory advantage that odors causing strong emotional reactions normally have.

Autobiographical memory after temporal lobe resection: neuropsychological and MRI volumetric findings

This study examined the contribution of medial temporal lobe (MTL) structures in autobiographical memory. While some investigators have reported a temporal gradient in memory performance, characterized by retrieval difficulties limited to recent periods of life [Squire and Alvarez (Retrograde amnesia and memory consolidation: a neurobiological perspective. Curr Opin Neurobiol 1995; 5: 169-77)], others have suggested that this impairment involves all life-time periods [Nadel and Moscovitch (Memory consolidation, retrograde amnesia and the hippocampal complex. Curr Opin Neurobiol 1997; 7: 217-27)]. In this study, autobiographical memory was assessed in 22 patients who had undergone a left (n = 12) or a right (n = 10) MTL resection for the relief of epileptic seizures and in 22 normal control participants. For this purpose, we used an autobiographical memory task (TEMPau, Piolino et al., 2003) across four time periods covering the subjects' entire lifespan. For each period, an overall autobiographical memory score (AM score) was obtained, from which a strictly episodic score (SE score), characterized by specificity and richness of details, was computed. For all events recalled, Remember responses justified by specificity of factual, spatial and temporal contents (jR responses) were measured using the Remember/Know paradigm. MRI volumetric analyses performed on the medial (i.e. hippocampus, temporopolar, entorhinal, perirhinal and parahippocampal cortices) and lateral temporal (i.e. superior, middle and inferior temporal gyri) lobe structures stated that the resection mainly included MTL structures. AM and SE scores were impaired in patients with right and left MTL resections as compared to normal controls across all time periods, reflecting the patients' particular difficulty in producing specific and detailed memories across all periods. This impairment was associated with poor autonoetic consciousness, revealed by the small number of jR responses across all periods. Results of correlation analysis between MRI volume measures of temporal lobe structures and autobiographical memory scores suggest that the right MTL structures are particularly responsive in reliving the encoding context regardless of remoteness. Our results support the bilateral MTL contribution to episodic autobiographical memory covering the entire lifespan, which is consistent with the multiple trace theory of MTL function [Moscovitch et al. (Functional neuroanatomy of remote episodic, semantic and spatial memory: a unified account based on multiple trace theory. J Anat 2005; 207: 35-66.)].

A volumetric MRI study of the hippocampus and the parahippocampal region after unilateral medial temporal lobe resection

Segmentation guidelines on high-resolution MRI designed to assess remaining volumes of the hippocampus and the parahippocampal cortices after medial temporal lobe (MTL) surgery could provide a useful tool to investigate the involvement of these anatomical regions in surgical outcomes and in human memory. For this purpose, we implemented an MRI volumetric analysis, already applied to healthy population or epileptic patient before surgery, to quantify the volume of the hippocampus, the temporopolar cortex and the regions of the parahippocampal gyrus (perirhinal, entorhinal and parahippocampal cortices) spared after unilateral MTL resection carried out to treat medically uncontrolled temporal lobe epilepsy (TLE). Based on the locations of remaining anatomical landmarks, we quantified the volume of these regions in 24 patients after MTL resection and in 16 control participants. Our results show that (1) mean volumes of these regions contralateral to the epileptic focus were similar to those of normal subjects, (2) volumetric measures obtained from the resected side were much smaller than those from the non-resected side or from normal values and (3) the extent of MTL resection was comparable in right or left MTL surgery. Individual analysis of patients showed that the parahippocampal cortex, as opposed to the other regions, was not systematically removed across patients. As a post-operative MRI-based method, it therefore proves valuable to assess group data as well as to explore differences between individual patients.

Loss of memory for auditory–spatial associations following unilateral medial temporal-lobe damage

The goal of the present experiment was to determine the role of medial temporal-lobe structures in episodic memory of auditory–spatial associations. By using a two-alternative forced choice paradigm in which an association between eight different sounds and their spatial location must be recognized, learning abilities over 10 learning sessions were tested in 19 patients who had undergone a right or a left medial temporal-lobe resection for the relief of intractable seizures as well as in nine normal control participants. The data demonstrated that significant learning took place over the successive sessions for all the participants. In addition, the results showed that patients with left but not right medial temporal-lobe lesion were impaired in this learning task as compared to normal participants, suggesting the predominant implication of left medial temporal-lobe structures in auditory–spatial associative learning. The predominant role of left hemisphere structures in this memory task could be explained by a spatial categorical coding, which was enhanced by the use of eight loud-speakers. This result also suggests that the ability to store an episodic event associated with a rich spatial (or temporal) context depends on the left medial temporal-lobe structures. Thus, this finding provides an interesting parallel with data obtained in the visual modality by documenting for the first time the role of the left medial temporal-lobe in episodic learning of auditory–spatial associations.